This invention generally relates to variable data rate transmissions and, more particularly, to techniques for efficiently detecting variable rate data transmission when explicit bit rate information is transmitted.
Cellular radio communication systems have recently been developed that use spread spectrum modulation and code division multiple access (CDMA) techniques. In a typical CDMA system, an information data stream to be transmitted is superimposed on a much-higher-bit-rate data stream sometimes known as a spreading code. Each symbol of the spreading code is commonly referred to as a chip. The information signal and the spreading code signal are typically combined by multiplication in a process sometimes called coding or spreading the information signal. Each information signal is allocated a unique spreading code. A plurality of coded information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal can be isolated and decoded.
As radiocommunication becomes more widely accepted, it will be desirable to provide various types of radiocommunication services to meet consumer demand. For example, support for facsimile, e-mail, video, internet access, etc. via radiocommunication systems is envisioned. Moreover, it is expected that users may wish to access different types of services at the same time. For example, a video conference between two users would involve both speech and video support. Some of these different services will require relatively high data rates compared with speech service that has been conventionally supplied by radio communication systems, while other services will require variable data rate service. Thus, it is anticipated that future radio communication systems will need to be able to support high data rate communications as well as variable data rate communications.
Several techniques have been developed to implement variable rate communications in CDMA radio communication systems. From the perspective of transmitting data at varying rates, these techniques include, for example, discontinuous transmission (DTX), variable spreading factors, multi-code transmission and variable forward error correction (FEC) coding. For systems employing DTX, transmission occurs only during a variable portion of each frame, i.e., a time period defined for transmitting a certain size block of data. The ratio between the portion of the frame used for transmission and the total frame time is commonly referred to as the duty cycle .gamma.. For example, when transmitting at the highest possible rate, i.e., during the entire frame period, .gamma.=1, while for zero rate transmissions, e.g., during a pause in speech, .gamma.=0. DTX is used, for example, to provide variable data rate transmissions in systems designed in accordance with the U.S. standard entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", TIA/EIA Interim Standard TLA/EIA/IS-95 (July 1993) and its revision TIA/EIA Interim Standard TIA/EIA/IS-95-A (May 1995). Such standards that determine the features of U.S. cellular communication systems are promulgated by the Telecommunications Industry Association and the Electronic Industries Association located in Arlington, Va.
Varying the spreading factor is another known technique for providing variable data rate communication. As mentioned above, spread spectrum systems spread data signals across the available bandwidth by multiplying each of the data signals with spreading codes. By varying the number of code symbols or chips per data symbol, i.e., the spreading factor, while keeping the rate of the code symbols fixed, the effective data rate can be controllably varied. In typical implementations of the variable spreading factor approach, the spreading factor is limited by the relationship to SF=2.sup.k.times.SF.sub.min where SF.sub.min is the minimum allowed spreading factor corresponding to the highest allowed user rate.
Another known technique for varying the transmitted data rate is commonly referred to as multi-code transmission. According to this technique, data is transmitted using a variable number of spreading codes where the exact number of codes used depends on the instantaneous user bit rate. An example of multi-code transmission is described in U.S. patent application Ser. No. 08/636,648 entitled "Multi-Code Compressed Mode DS-CDMA Systems and Methods", filed on Apr. 23, 1996, the disclosure of which is incorporated here by reference.
Yet another technique for varying the transmitted data rate in radio communication systems involves varying the FEC. More specifically, the rate of the forward error correction (FEC) coding is varied by using code-puncturing and repetition or by switching between codes of different rates. In this way the user rate is varied while the channel bit rate is kept constant. Those skilled in the art will appreciate the similarities between varying the FEC and a variable spreading factor as mechanisms to implement variable rate transmission.
Regardless of the particular technique adopted in a radiocommunication system for providing variable rate transmission capability, the receiver must know the particular data rate at which a signal is transmitted in order to properly detect and decode the received signal. Methods for informing the receiver of the instantaneous data rate of a received signal generally fall into two categories, systems which explicitly transmit bit rate information (BRI) along with the transmitted signal, and systems which provide the receiver with the capability to "blindly" determine the rate at which data has been transmitted, e.g., by trying different rates and looking for a correct cyclic redundancy check (CRC). U.S. Pat. No. 5,566,206 to Butler et al. provides an example of blind rate detection.
Both the transmission of explicit BRI and blind rate detection approaches have certain drawbacks. For example, blind rate detection results in relatively complex receivers due to the additional circuitry/logic needed to correctly identify one of a plurality of possible data transmission rates.
The transmission of explicit BRI also creates design issues. For example, the BRI can either be sent in the data frame before the data frame that it describes or in the same frame that it describes. If the BRI is transmitted in the previous frame, an extra delay of one frame will be introduced in the transmitter. That is, as soon as the data for a frame is available in the transmitter, the BRI for that frame is computed and transmitted, while the transmission of that data frame is delayed until the next frame period. This extra delay can be undesirable for low-delay services like speech, especially for large frame lengths.
On the other hand, if the BRI is transmitted in the same frame as the data, the receiver needs to buffer the received signal until it has detected and decoded the BRI for that frame. This solution leads to extra buffering in the receiver, and therefore additional cost and complexity.
Accordingly, it would be desirable to create new techniques and systems for allowing explicit rate information to be transmitted in the same frame that it describes, while minimizing the amount of buffering needed in the receiver.